Embodiments of the present invention relate generally to diagnostic imaging and, more particularly, to a system and method of dual energy spectral imaging having minimized noise.
Medical imaging devices comprise x-ray systems, magnetic resonance (MR) systems, ultrasound systems, computed tomography (CT) systems, positron emission tomography (PET) systems, nuclear medicine, and other types of imaging systems. Typically, in CT imaging systems, an x-ray source emits a fan or cone-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis, which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry opening within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom. Such typical systems, however, do not include an ability to discriminate spectral energy content of x-rays as they pass through an object being imaged.
However, as known in the art, dual or multi-energy spectral CT systems have been developed that can reveal the densities of different materials in an object and generate images acquired at multiple monochromatic x-ray energy levels. In the absence of object scatter, a system derives the behavior at a different energy based on a signal from two regions of photon energy in the spectrum: the low-energy and the high-energy portions of the incident x-ray spectrum. In a given energy region of medical CT, two physical processes dominate the x-ray attenuation: (1) Compton scatter and the (2) photoelectric effect. The detected signals from two energy regions provide sufficient information to resolve the energy dependence of the material being imaged. Detected signals from the two energy regions provide sufficient information to determine the relative composition of an object composed of two hypothetical materials.
Different approaches have been developed to realize dual energy or spectral imaging. To name a few, dual x-ray source and detector, single x-ray source and detector with multiple acquisitions at different kVp or interleaved with fast kVp switching capability, and single x-ray source with an energy discriminative detector, are leading techniques.
In a dual x-ray source and detector system, typically two x-ray sources are provided, each having a respective detector positioned opposite thereto such that x-rays may be emitted from each source having a different spectral energy content. Thus, based on the known energy difference of the sources, a scintillating or energy integrating device may suffice to distinguish energy content and different materials within the object being imaged.
In a single x-ray source and detector arrangement, a conventional third generation CT system may acquire projections sequentially at different peak kilovoltage (kVp) levels, which changes the peak and spectrum of energy of the incident photons comprising the emitted x-ray beams. Two scans are acquired—either (1) back-to-back sequentially in time where the scans require two rotations around the subject, or (2) interleaved as a function of the rotation angle requiring one rotation around the subject, in which the tube operates at, for instance, 80 kVp and 140 kVp potentials.
In a single x-ray source with an energy discriminative detector, energy sensitive detectors may be used such that each x-ray photon reaching the detector is recorded with its photon energy. Such systems may use a direct conversion detector material in lieu of a scintillator.
Once dual or multi-energy data is obtained, a basis material decomposition (BMD) algorithm may be applied in order to image two distinct materials, such as water and iodine, as examples. A conventional BMD algorithm is based on the concept that, in an energy region for medical CT, the x-ray attenuation of any given material can be represented by a proper density mix of two materials with distinct x-ray attenuation properties, referred to as the basis materials. The BMD algorithm computes two material density images that represent the equivalent density of one of the basis materials based on the measured projections at high and low x-ray photon energy spectra, respectively. The material density images may be further converted to form monochromatic images at other desired monochromatic energies.
However, noise in the material density images and monochromatic images is propagated during the decomposition process, and the noise is typically correlated. In other words, noise generated in both low and high kVp acquisitions typically correlates during material decomposition (MD) and propagates in subsequent generation of basis material images. Known noise reduction schemes are available, however, such schemes are typically limited to reducing negative correlated noise.
Therefore, it would be desirable to have a system and method that presents diagnostic imaging data having minimized noise in computed monochromatic images and material density images.